Cities are the once and future centres of agriculture. That’s true in spades for cities that are chic and sophisticated, and chock-a-block with people.

Paris until a century ago was an agricultural powerhouse, producing some 110 pounds of produce per person per year in the area known as Le Marais, now a trendy district of residences and museums. Other major cities, on both sides of the Atlantic, were also large food producers, meeting much of their own food needs and exporting their specialties to each other. But Paris was in a class by itself, not only because of the scale of its operations – one-sixth of its area was turned to food production – but also because it perfected the high-input, high-profit, niche farming phenomenon known as la culture maraichere. During its heyday from the 1850s to the late 1880s, a period during which its population doubled from one to two million, this intensive form of market gardening produced more than 100,000 tons a year of lucrative, out-of-season salad crops as well as other vegetables, fruits and flowers. Much of this cultivation employed advanced greenhouse technologies, occurring under glass-covered frames and bell-shaped glass cloches that trapped the warmth of the sun.

London, a more populous city, was less of an agricultural power, not because it lacked technology – England had launched the Industrial Revolution – but because it lacked an essential present in Paris: an appetite for fine food in a great number of citizens willing to pay a handsome premium for it. In the Paris market, competition for fresh produce was so acute that, for example, a mere one-day delay in getting apricots to market cost one-seventh the sale price. This demand for culinary excellence drove innovations that let the Paris maraichers overcome the chief impediments to farming in the city, namely the high cost of urban land and other inputs.

The maraichers’ solution lay in replacing the technology of the farmer – the plow, the horse – with that of the gardener. To get more produce out of each tiny plot, Parisian gardeners germinated seeds separately, then transplanted them into beds of loam, often under glass, to develop their rootlets. Sickly plants would be discarded and successful ones bedded in the scarce, open ground, where they would be tended until ready for next day, and often same day, consumption. Using this intensive method of cultivation, Paris gardeners obtained three to six growing seasons in a city that had been considered “two degrees of latitude” too north to grow vegetables year round.

The heat needed to emulate the soils of more southerly climes came only partly from the sun. Horse manure – 100,000 tons of it a year, much of it from the city’s extensive horse-drawn public transit system – not only formed new soil and provided nutrients but, more importantly, warmed the soil as it fermented. Carbon dioxide from the fermenting manure also aided plant growth. Water, a commodity often scarce in cities, came from the sewers, pumped by steam engines through the maraichers’ personal irrigation pipes.

The rise of public transit, in effect, fuelled an urban farm industry, but improvements in public transit also spelled the farm industry’s demise. When motorized vehicles replaced the horse-drawn car, urban agriculturalists lost their cheap fuel and fertilizer. Coupled with the high cost of city land, urban farming was doomed. By World War I, Paris’s once plentiful market gardens – 1,800 of them on 3,500 acres at their peak – had all but disappeared. In other western cities, urban farming also died out.

Yet urban agriculture has never lost its potential, as can be seen in affluent Hong Kong, the world’s densest city. Because Hong Kong’s Chinese population places a high premium on fresh produce and freshly killed animals, the city produces two-thirds of the poultry, one-sixth of the pigs, and almost half of the vegetables eaten by its citizens and visitors on about 5% of the city’s land. Dense, affluent Singapore, which even farms between highrises, similarly shows agriculture’s viability: Singapore’s 10,000 urban farmers produce 80% of the poultry and one-quarter of the vegetables consumed locally.

Without the zoning laws that now outlaw agriculture in western cities, and without the subsidies to inefficient rural agriculture, many of our cities, too, would see farms on underused lands. Urban farming could be ubiquitous in cities, including in dense downtowns, by tapping into free energy, free water, and free land that now goes wasted.

The city’s abundance of plots, suitable for growing fruits, vegetables, and flowers, are found atop its office towers, condos, and apartment buildings. There reside the buildings’ heating and cooling systems which are often placed on roofs to dispose of their waste heat, among other reasons. Greenhouses located next to this roof equipment, or on platforms built above the equipment, could capture an inexhaustible amount of waste heat year-round, giving them a competitive advantage over rural greenhouses, whose energy costs are often their single-biggest operating expense. Rooftop greenhouses, as well as open-air plant beds, would have other advantages, too. Their produce could be delivered by elevator directly to stores and restaurants at ground level, and to customers within the buildings, rather than requiring trucking and warehousing. Unlike rural farms, which often vie for water with residential and recreational users, urban farms would tap into plentiful supplies of treated “grey water” – the waste water from kitchens, washrooms, or other facilities suitable for irrigating plants. Pumping grey water up to roof nurseries instead of disposing of it by sewers, and diverting rainwater to roof crops instead of carrying it down pipes below ground, would also relieve pressure on municipal water works. Only one major impediment prevents rooftop farms: urban zoning laws that ban commercial agriculture in cities. Rescind the laws and plantations would become a fixture of the urban skyline.

As it is, many high-end restaurants already plant at roof level – it’s legal to do so if they use their crops in their own operations – and hydroponic gardening and balcony crops have become a billion-dollar industry. Vegetation atop buildings provides environmental benefits. In many European cities, these “green roofs” are used to insulate buildings from heat and cold, cleanse air of pollutants, and prevent roof runoff from entering the sewer systems. These amenities also save money for the building owners and for society as a whole, largely justifying the expense of the green roof installation and convincing many North American cities to join the trend. One more amenity – fresh, high quality produce – would put the economic calculation over the top.

This is last of a series; Lawrence Solomon is executive director of Urban Renaissance Institute and Consumer PolicyInstitute, divisions of Toronto-based Energy Probe ResearchFoundation. www.urban.probeinternational.org